Rock core samples are extracted and analysed in a variety of industries. A rock core sample extracted from below ground can be used to obtain detailed information about the formation from which it originated. These samples are often analysed using imaging or spectroscopy techniques, such as magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) spectroscopy. Typically, rock core samples are cylinders with a fixed diameter and flat parallel ends. Such samples are often subjected to high pressure and temperature in their native environment, which must be reproduced in the laboratory for optimal analysis.
In petrophysical research applications, a range of parameters can be measured from core samples during the relatively low-cost initial drilling stage to assist in the identification of “sweet spots” suitable for further exploration. The depth of wells used by the hydrocarbon industry continues to increase, leading to down-hole conditions having very high pressure and temperature. Accordingly, laboratories need instrumentation capable of reproducing these extreme conditions. Improvements in the correlation between data collected down-hole with that measured in a controlled laboratory setting can improve the targeting of “sweet spots” and can have a major impact on the economics of production.
Various types of sample holders can be used during analysis to attempt to simulate underground conditions. Preferably, core sample holders employed in the laboratory will apply pressure to the outside of the rock core at temperatures equivalent to the down-hole conditions. While under pressure, a secondary fluid that is detectable by the spectroscopic technique being used is typically forced into or through the rock core to allow the relevant parameters to be determined. Such sample cells are commonly called overburden cells since they apply pressure equivalent to the overburden experienced by the rock core underground.
There are several examples of sample core holders for the study of geologic cores in the literature. Sample core holders can be classified based on how pressure is applied to the core sample. A uniaxial core holder, or Hassler core holder, has a single inlet for the application of pressure to the core sample. This type of holder might be used to measure the pressure drop along the length of the core during flooding experiments. A biaxial core holder provides for two independent and isolated pressure sources. One source leads directly to and through the core and is typically the fluid of interest in analysis. The secondary source provides the confining pressure on the core to simulate the below ground conditions. This source acts on the core both axially, through the mounts that are in direct contact with the core faces, and radially around the core, through some type of compression sleeve. A triaxial core holder uses three independent pressure sources. One source is for delivering fluid through the core, one is for delivering pressure to the axial faces of the core sample, and another is for delivering pressure radially to the core (see, e.g., Brauer et al., U.S. Pat. No. 4,599,891; Reed et al., U.S. Pat. No. 4,753,107).
Some types of sample holders can be used with NMR spectroscopy, which requires materials that are non-magnetic and non-metallic, at least in the region surrounding the core sample (see, e.g., Vinegar et al., U.S. Pat. No. 4,827,761). For many modern, commercial NMR spectrometers there is a defined bore diameter of the instrument which cannot be easily altered. Therefore, most core holders are limited in the outside diameter, which is often not much larger than the geologic core sample. This limits the ratio of outside and inside diameters, which often largely defines the maximum pressure that can be reached. Accordingly, these limitations can result in the housing wall being relatively thin, which greatly reduces the ability of the housing to resist the internal pressure being applied. Further, fastening end plugs to the housing can be problematic if the housing wall is relatively thin.
Several methods have been devised to overcome these issues. One such method uses a large external clamping system that resembles a hydraulic press to hold the end plugs inside the housing. This method is reasonably successful, provided the end plugs can be very accurately aligned axially both with the housing and the direction of force applied by the clamps. Failure to achieve this alignment makes the system prone to leaks and can severely limit the maximum operating pressure. Another method uses pins applied radially through the wall of the housing to fasten the end plug to the housing. Although this type of sample holder may not suffer from the alignment issues of the previous method, it can be difficult to assemble and disassemble. Notably, current commercially available core holders for NMR make use of fiberglass or composite plastic as the material for at least the part of the housing surrounding the sample itself. These types of sample holders are useful for lower pressure, but are not capable of addressing the current pressure and temperature requirements of geological and petrophysical research.
Thus, there is a continuing need in the art for a core sample holder that can withstand the high temperatures and pressures associated with current petrophysical research, and that can also be used with NMR spectroscopy or MRI analysis. The present invention addresses this continuing need in the art.